A. Field of the Invention
The invention relates to scanners for providing a digitized image of a document and, more particularly, to a high quality but inexpensive rotary scanner.
B. Prior Art
Scanners are increasingly utilized for digitizing both materials such as letters, memoranda, photographs, or the like, as well as transparencies; for ease of reference, these will all be referred to hereinafter simply as "documents" or "a document".
There are two principal types of scanners, namely, flat bed scanners and rotary scanners. Flat bed scanners mount the document on a flat surface or "bed" and scan the document from side to side and line by line to form the desired image which is then in a form suitable for high speed processing. The scanning apparatus in flat bed scanners is often complex and requires careful adjustment in order to maintain image quality. Further, the scanning resolution is often limited both in resolution and in dynamic range by the nature of the linear array used for detection, and thus scanners of this type are found most often in the "low end" price range of scanners.
Rotary scanners, in contrast, mount the document on a drum and provision is made for relative motion between the drum and a scanning head which forms the digitized image. Because the relative motion is primarily rotational in nature, high scanning rates can be achieved, and systems of this type are commonly used in commercial operations. However, they are generally much more expensive than flat bed scanners, and this has restricted their use.
Quality scanners typically use high intensity light sources to illuminate the source being scanned. These light sources generally have a relatively short life, and thus require frequent replacement. Because the alignment of the light source within the optical system frequently is critical, the replacement is generally performed by a trained technician from the scanner manufacturer, and this increases the cost of maintenance of the system.
In order to efficiently collect light reflected from, or transmitted through, a document, a lens of high numerical aperture is desirable. However, such lenses have a very narrow depth of focus, and the common variation of document thickness from source to source can cause unacceptable variations in focus.